There is no buoyancy yet, so it isn't possible currently. Not sure about what can happen in the future however.
Buoyancy shouldn't be too hard to simulate. However, proper water physics (ie preventing water from just clipping into the mesh) aren't easy at all, and since this game isn't based on water physics I don't see that happening anytime soon. However, basic buoyancy should be a thing and yes, you could make boats and even amphibious cars (want).
